1. The Field of the Invention
The present invention relates to a thermocatalytically active coating, a substrate which has a thermocatalytically active coating at least in parts, and also a process for producing a thermocatalytically active substrate surface and use thereof.
2. The Related Art
Thermocatalytic layers and surfaces are generally known and are used for car exhaust cleaning and for supporting self-cleaning in baking ovens. Generally, such coatings are noble metal coatings, such as platinum, or enamel coatings which contain transition metals. Baking ovens comprise a closable cooking space which is delimited by a baking oven muffle. In use, the side walls of this cooking space and also the inspection glass are frequently fouled, for example by fat splashes and roasting juices. Since this fouling is usually unavoidable, various attempts have already been undertaken to facilitate cleaning the cooking space. Generally, a differentiation is made here between catalytic and pyrolytic cleaning.
In pyrolytic cleaning of baking ovens, heating elements provided as extra therefore are activated, which elements ensure that the fouling is completely burnt off in about 2 h at temperatures of 500° C. This pyrolytic cleaning is complex because of the high temperatures which must be used and expensive owing to the constantly increasing energy costs.
Cleaning inspection glasses of combustion chambers, such as flue stoves, for example, is similarly complex to arrange. Here also cleaning the inspection glasses is currently only achievable using pyrolytic methods. However, since the manufacturers of flue stoves seek to avoid high temperatures at the inspection glasses for reasons of efficiency and safety, generally the temperature of 500° C. which is required for a pyrolytic cleaning is not reached at all or at least not over the entire surface.
In the case of catalytic cleaning, the decomposition and oxidation and/or the combustion of fouling and gaseous exhaust gases proceed by the aid of catalytic and/or oxidizing agents even at low temperatures, i.e. temperatures significantly below 500° C. Therefore, seen from the cost aspect, catalytic systems are preferred over the pyrolytic systems. In the case of catalytic cleaning systems, the surface to be cleaned is generally provided with a catalytically active coating.
Catalytic layers in baking ovens are known as coatings of the baking oven muffle. In DE 101 50 825 A1 a high-porosity coating is described which consists of porous particles and a binder matrix. The object of this thick coating is to spread out and absorb the fats in the layer. The catalytic decomposition proceeds in the presence of transition metal particles and can start even at temperatures <300° C. These prior art coatings are not transparent. For this reason, to date the baking oven inspection glass has not been coated with self-cleaning catalytically active coatings, and thus the baking oven glass still had to be cleaned with abrasive agents or other chemical cleaners.
U.S. Pat. No. 7,297,656 B2 describes catalytic layers which degrade diesel soot. The coating consists of doped platinates. In the description, dopings of platinates with lithium are described in which the start temperature of the catalytic reaction of platinum is lowered from 392° C. to 366° C. In this patent the presence of platinum is essential. It is not obvious to a person skilled in the art that layers having lithium without platinum could exhibit outstanding activity.
EP 0 645 173 B1 also describes a catalyst of a combination of lithium and platinum or palladium. The lithium is intended to prevent poisoning reactions of the platinum or palladium. Lithium is produced in an Al2O3 support in a composite as strong as possible onto which the catalyst platinum or palladium and also other metal ions are then applied. This type of catalyst is used for exhaust gas purification. Here also it is not clear to a person skilled in the art that lithium without platinum or the remaining composite of the described support materials will be active.
Alkali metals, preferably potassium, are described in EP 1 412 060 B1 in use in spark-ignition internal combustion engines. Here, support materials are impregnated with aqueous potassium solutions. In this patent there is no assumption that transparent products will become possible. Also, in the description it is stated that the catalytic action is obtained primarily from the redox potential of the support materials, with the effect being yet further improved by the alkali metals and here, primarily, potassium. Lithium as sole active component is not described.
A further filter for diesel soot may be found in EP 0 105 113 B1. In the patent, as an exemplary embodiment an aqueous LiOH solution is poured over a filter. The ignition temperature is 467° C. On account of the high ignition temperature disclosed in the patent publication, it cannot be assumed that the composition selected can be utilized for the present invention. In addition, LiOH is highly alkaline and aggressive and corrosively attacks most substrate materials such as, e.g., glass, glass ceramic, metal and some plastics.
Catalytic layers are also used for deodorization.
EP 1 165 153 B1 describes silicate layers, the catalytic activity of which is achieved by adding transition metal particles. Optionally, in the description, the addition of glass-forming elements is described among which, inter alia, lithium is also mentioned. Since lithium is known as a glass-forming element and lithium-containing glasses without an appropriately finished surface do not have catalytic activity, it is not obvious that the lithium here is catalytically active.
The use of noble metals is cost-intensive and frequently has the disadvantage, as do the coatings of transition metal compounds, that the layers have an inherent color. In addition, materials having very high surface areas are frequently required in order to obtain a sufficient degradation reaction rate.
There is therefore a need for more durable and longer-life, transparent surfaces capable of self-cleaning. These are advantageous, especially, in combustion chambers, such as, for example, flue stoves, or else in cooking, roasting, baking and grilling devices, and also in components for flue gas purification, such as, for example, odor-reducing components.